Return cache export errors of dep when mode is max.

[sipsma]

@tonistiigi I'm 99% sure this isn't correct as is, just sending out for discussion.

My best guess is that the right behavior is that when mode=max we are attempting to export every record and should thus fail if anything fails.

When mode=min it looks like the code intends to try to include the export if we already have a remote for it, thus these lines: https://github.com/sipsma/buildkit/blob/a33bb54df091a95350ff1a05278eb5f9bfe24a6a/solver/exporter.go#L164-L166

? Not totally sure on that front. If it's true, then I can see why ignoring the error makes sense in that case.

Let me know if that's on the right track, happy to update further with specific error type checks and such too.

[tonistiigi]

So this errors on ModeMin? What is that error?

[sipsma]

From what I can tell (and I might be wrong, worth cross-checking) the calls that can return an error when recursing to deps while mode is min are:

1. buildkit/solver/exporter.go

   Line 114 in a33bb54

   res, err := cm.backend.Load(key, v.ID)

2. buildkit/solver/exporter.go

   Line 119 in a33bb54

   remotes, err := cm.results.LoadRemotes(ctx, res, opt.CompressionOpt, opt.Session)

3. buildkit/solver/exporter.go

   Line 223 in a33bb54

   if err := cm.backend.WalkIDsByResult(v.ID, func(id string) error {

4. buildkit/solver/exporter.go

   Line 28 in a33bb54

   if err := cm.backend.WalkBacklinks(id, func(id string, link CacheInfoLink) error {

Now that I've actually looked through those various methods, it seems like they all only return an error when something wrong really happened and ignore them otherwise. So given that, maybe it's actually better to just always unconditionally return the error in every mode after all?

[sipsma]

Also, to be clear, I have not hit any errors here in the real world. I only noticed this issue because I was reading through the code to get a better understanding of it.

[sipsma]

I ended up hitting real-world problems caused by this, closing in favor of #5560